Influenza virus infections contribute yearly up to 40,000 human deaths in the U.S. alone. This virus could cause much more deadly pandemics through novel reassortants when pre-existing immunity does not exists. Humoral responses to influenza virus contributed by B cells form a major component of immune-mediated short and long-term protection. Much of our knowledge regarding the regulation of B cell responses comes from in vivo and in vitro analyses of anti-protein or hapten-carrier injections. However, increasing evidence, provided in part by studies conducted during the initial funding period of this grant, suggests that immediate early exposure of B cells to infection-induced innate signals shape the responses of B cells. Little is known about the nature of these signals and the mechanisms by which they affect the B cell response. The working hypothesis to be tested here is that infection-induced local innate immune signals differentially regulate various B cell subsets involved in the induction of protective immunity to influenza virus infection. The long-term objective of the studies is to determine how respiratory tract immunity to viral infections is induced and regulated. The objective of this proposal is to determine the mechanisms by which innate immune signals, particularly type I IFN shape the quality and magnitude of antiviral B cell responses. This is based on studies during the last funding period which showed type I IFN as a major infection-induced B cell stimulus in regional lymph nodes within 2 days of infection. To achieve our objective three Specific Aims will be carried out. Specific Aim #1 will determine the mechanisms by which direct IFNR-signals received by B cells affect the magnitude and protective capacity of individual B cell response components to influenza: B-1 cells, extrafollicular foci and germinal center responses. Specific Aim #2 will determine the role of toll-like receptor (TLR) 3 and 7-mediated signals on antiviral B cell response regulation to influenza infection and their integration with stimuli provided by the B cell receptor and/or T cell help. In Specific Aim #3 the effects of IFNR-mediated B cell stimulation on local CD4 T cell responses to influenza virus infection and particular the affects on CD40-CD40L mediated help will be investigated. In vitro and in vivo tests are aided by the use of virus-specific T cell receptor transgenic mice. Completion of these studies will contribute to a better understanding of the processes that regulate the induction of protective antiviral B cell responses to influenza virus.